What an awesome idea! Most people just slap bumper stickers on their cases, which I've never cared for. But painting the case... I'm envisioning a mini-mural, an airbrushed design like some folks put on their cars and motorcycles, painted on a smooth plastic case. Imagine the "oooh's" and "ahhh's" you'll get! (Airbrushing probably wouldn't work on my fabric gig bag, though...)

Leeneia makes a good suggestion about going to a paint store first. Beware, though, because they may be more interested in selling you something than in giving you sound advice. If you're in a community that has artists' studios, an art gallery, a college that offers art courses, or a public arts center, I think you might want to consult with some of their knowledgable artists and/or art instructors, too.

I imagine that there must be a lot of good painting tips on the internet! I have to go to a rehearsal soon but I'll try to search out some sites later.